Self-excited microelectromechanical device

ABSTRACT

A self-excited microelectromechanical device is described. The device includes a resonating structure, such as a cantilever, which responds to a physical phenomenon by generating an induced variable frequency voltage signal corresponding to the physical phenomenon. Self-excitation circuitry connected to the cantilever processes the induced variable frequency voltage signal and produces a variable frequency voltage signal in a resonant pass band of interest that is applied to the cantilever to augment the effect of the physical phenomenon on the cantilever. An exemplary use of the device is as a power line sensor. In this context, the cantilever responds to the electric field associated with a power signal on a power line. The cantilever transforms the voltage signal of the electric field into a corresponding frequency signal. The noise-immune frequency signal can be readily processed to reconstruct the power signals carried by the power line.

BRIEF DESCRIPTION OF THE INVENTION

This invention relates generally to microelectromechanical devices, alsoknown as micromachines. More particularly, this invention relates to aself-excited microelectromechanical device which operates as avoltage-to-frequency converter which can be used, for example, as alow-power sensor to monitor power signals on a power line.

BACKGROUND OF THE INVENTION

Microelectromechanical devices (also called micromechanical devices ormicromachines) are small (micron scale) machines that promise tominiaturize instrumentation in the same way microelectronics haveminiaturized electronic circuits. Microelectromechanical (MEM) deviceshave configurations analogous to conventional macroscale machinery, suchas cantilevers, motors, and gear trains.

The present invention is related to MEM cantilever devices, andanalogous resonating structures. It is known to use the deflection of aMEM cantilever device to measure physical phenomena, such as smallmechanical pressure variations associated with sound. A patentapplication entitled "Cantilever Pressure Transducer", Ser. No.08/072,294, filed Jun. 10, 1994, and assigned to the assignee of thepresent invention, discloses one such device.

It would be highly desirable to provide a MEM cantilever device with thecapability of measuring different physical phenomena. Such a devicewould enable the exploitation of other benefits of MEM devices, such astheir compactness and low power consumption.

If the structure of a MEM cantilever device was suitably improved, itcould be used, for example, to reliably measure an electric fieldassociated with a power line. This would potentially provide a compactand low-power power line sensor.

Electric utilities are interested in monitoring power parameters, suchas current and voltage, along the length of powerlines. This informationis useful in locating powerlines that have failed, are failing, or havesome other undesirable attribute, such as arcing activity, voltage sag,momentary outage, improper loading, or poor "power quality" as evidencedby the presence of harmonics of the power line frequency.

Shortcomings associated with existing power line sensors make itdifficult to provide comprehensive power line monitoring. Existing powerline sensors are relatively expensive because they require insulation ofthe current or voltage transducers for the full electric potential ofthe power line. The insulation is expensive and bulky. In addition,existing sensors typically require a connection to ground, therebynecessitating a physical electrical connection from the power line tothe ground. Consequently, existing power line sensors are inherentlyexpensive, bulky, and expensive to install. It would be highly desirableto provide a power line sensor which could operate without theinsulation and ground connection associated with prior art power linesensors. Such a device would be compact and relatively simple toinstall. Preferably, such a device would be inexpensive and have lowmaintenance requirements.

Improvements in power sensing are also desirable for watt and watt-hourmeters at customer locations. It would be highly desirable to provide acompact, reliable, inexpensive solid-state power meter.

SUMMARY OF THE INVENTION

The invention is a self-excited microelectromechanical device. Thedevice includes a resonating structure, such as a cantilever, whichresponds to a physical phenomenon by generating an induced variablefrequency voltage signal corresponding to the physical phenomenon. Theself-excitation circuitry causes the cantilever to vibrate at one of itsresonant frequencies. This frequency is affected by the instantaneouselectric force between an electrode on the cantilever and a nearbyelectrode.

An exemplary use of the self-excited cantilever device is as a powerline sensor. In this context, the cantilever responds to an electricfield associated with either the voltage waveforms or the currentwaveform on a power line. The cantilever transforms the electric fieldinto a corresponding frequency signal. The noise-immune frequency signalcan be readily transmitted and processed to reconstruct the voltage orcurrent waveforms of the power line.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of the invention,reference should be made to the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. 1 illustrates one embodiment of the microelectromechanical deviceof the invention.

FIG. 2 is a side view of one embodiment of the microelectromechanicalcantilever of the invention.

FIG. 3 is a top view of one embodiment of the microelectromechanicalcantilever of the invention.

FIG. 4 is a perspective view of one embodiment of themicroelectromechanical power line monitoring apparatus of the invention.

FIG. 5 is a top view of one embodiment of the microelectromechanicalpower line attachment assembly of the invention.

FIG. 6 is an axial view of the assembly of FIG. 5.

FIG. 7 is a schematic of one embodiment of the printed circuit boardused in accordance with the microelectromechanical power line attachmentassembly of the invention.

FIG. 8 is a detailed schematic of a number of the components of thedevice shown in FIG. 7.

FIG. 9 is a schematic of one embodiment of the microelectromechanicalbase station of the invention.

FIG. 10 illustrates voltage-to-frequency conversion values that may beused in accordance with the invention.

FIG. 11 is a power meter that may be used in accordance with theinvention.

FIG. 12 illustrates a MEM device and processing circuitry that may beused in the power meter of claim 11.

FIG. 13 is a schematic block diagram of a measurement resolutionenhancement device which may be used in accordance with the invention.

FIG. 14 illustrates an alternate embodiment of the self-excitedmicroelectromechanical device of the invention.

FIG. 15 illustrates another alternate embodiment of the self-excitedmicroelectromechanical device of the invention.

FIG. 16 illustrates still another alternate embodiment of theself-excited microelectromechanical device of the invention.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates one embodiment of the self-excitedmicroelectromechanical (MEM) device 20 of the invention. Theself-excited MEM device 20 includes a MEM device 22, self-excitationcircuitry 24, and an output device 26 (an antenna in this embodiment).The MEM device 22 includes a resonating structure. The resonatingstructure may be a cantilever 28 with an associated plate, shown as aMEM plate 30 in FIG. 1. The MEM plate 30 has a MEM plate input node 32and a MEM plate common node 34. The MEM plate input node 32 is connectedto an electrically conducting plate 36, which is positioned on anon-conducting spacer 35.

By way of example, the invention is disclosed in relation to itsimplementation as a power line sensor. In this context, a voltage (orcurrent) waveform derived from the power line line-to-line voltage (orline current) is applied to the MEM plate input node 32. Moreparticularly, as will be described below, the MEM plate input node 32receives a voltage (or current) waveform from a field plate (or currentcoil). The MEM plate common node 34 is electrically the same as thecantilever common 40, which is electrically connected to the power lineconductor.

The cantilever common 40 is positioned on a cantilever substrate 42. Asshown in FIG. 1, the region between the cantilever 28 and the MEM plate30 defines a device gap 44.

When a voltage signal (obtained from a field plate or a coil thatcouples magnetically to the power line current) is applied to the MEMplate input node 32, an electric field is induced in the device gap 44.This electric field deflects the tip of cantilever 28. Cantilever 28 isconfigured such that it is normally oscillating. In the exemplaryembodiment described herein, the deflection of the cantilever tip by theelectric field causes the resonant frequency of the cantilever 28 tochange. In the context of an AC voltage signal, the resonant frequencyof the cantilever 28 with no applied voltage will occur twice at thezero crossing of the applied signal each cycle. This reference point canbe used for various calibration functions. Calibration functions canalso be facilitated by placing an additional electrode on thecantilever. The additional electrode can monitor the electric field atthe cantilever and make use of this information during calibration.

The MEM resonator of the invention, such as a cantilever 28, isself-excited by circuitry 24 so that the resonator oscillates at aresonant frequency. Cantilever 28 carries two separately definedelectrodes 46 and 52. Electrode 46, together with ground plane 40 andpiezoelectric layer 47, comprises a feedback transducer whose outputvoltage is led by feedback line 48 to self-excitation circuitry 24. Theoutput of the self-excitation circuitry 24, an amplified variablefrequency voltage signal, is applied to drive line 50, which isconnected to a drive transducer defined by electrode 52, ground plane40, and piezoelectric film 53. If the gain of the amplifier in circuitry24 is appropriate, self-excited oscillation of the cantilever willresult, and the oscillation frequency will be affected by the electricfield existing in device gap 44.

The measurements made by the cantilever 28 may be processed by outputdevice 26, which is connected to drive line 50 by output line 51. Theoutput device 26 may be implemented in a number of manners. For example,the output device may be a micro-speaker with an amplifier thatamplifies the sound generated by the oscillating cantilever 28. In thiscontext, variations in sound from the cantilever correspond to frequencyshifts in the oscillation of the cantilever, which correspond tovariations in the voltage associated with the electric field in thedevice gap 44. The output device may also be implemented, for example,as an antenna. In this configuration, a variable frequency voltagesignal can be applied directly to the antenna to electromagneticallyinduce a corresponding signal in a remotely positioned antenna.

Attention now turns to FIG. 2, which illustrates a side view of a secondembodiment of a cantilever 28 in accordance with the invention. Thecantilever 28 may be implemented with a top metal layer forming groundplane 40' and a bottom metal layer forming electrode 46' and electrode52' sandwiching a piezoelectric thin film 64. In one embodiment, theground plane 40' was formed of 0.3 microns of aluminum, thepiezoelectric thin film 64 was formed of 0.5 microns of zinc oxide, andelectrode 46' and electrode 52' were formed of 0.2 microns of aluminum.In this embodiment, the cantilever substrate 42 was formed of 1.8microns of low-pressure chemical-vapor-deposited (LPCVD) low-stresssilicon nitride. Techniques for forming a device of this type aredescribed in the patent application, assigned to the assignee of thepresent application, entitled "Cantilever Pressure Transducer", Ser. No.08/072,294, filed Jun. 10, 1994, which is expressly incorporated byreference herein.

The foregoing dimensions result in a cantilever thickness, marked byarrow 68, of 2.8 microns. The implemented device of FIG. 2 had a length,marked by arrow 66, of 2000 microns. The width of the device, marked byarrow 70 of FIG. 3, was implemented as 2000 microns. This configurationresulted in a cantilever one of whose resonant frequencies fell between12 and 14 kHz.

FIG. 3 is a top view of the cantilever 28, showing the top metal layer.The figure illustrates that the top metal layer may be configured toinclude a metal region forming electrode 46 connected to line 48. Thetop metal may also include a metal region forming electrode 52 connectedto line 50. The bottom metal layer 40 is connected to node 34 (shown inFIG. 1).

The MEM plate 30 may be implemented from an electrically conductingplate 36 bonded to a non-conducting spacer 35 used to form device gap44.

As previously indicated, the disclosed MEM device 22 is configured for aresonant frequency of between 12 and 14 kHz. Those skilled in the artwill recognize a number of alternate configurations to achieve thisresonant frequency and other resonant frequencies of interest.

The self-excited MEM device 20 of the invention may be used in a varietyof contexts, but by way of example, the device will initially bedescribed as a power line sensor. FIG. 4 illustrates a power line pole80 which supports powerlines 82A, 82B, and 82C. Connected to thepowerlines are MEM power line attachment assemblies 86A, 86B, and 86C.Connected to the power line pole 80 is a MEM base station 88.

FIG. 5 is a top view of one embodiment of a MEM power line attachmentassembly 86 in accordance with the invention. The assembly 86 includes aMEM power line system housing 90 that supports a first field plate 92and a second field plate 94. Each field plate is a metallic plate whichaccumulates an electrostatic charge corresponding to its position inrelation to the power line 82. As will be described below, thedifference in electrostatic potential between field plate 92 and fieldplate 94 is used to generate an operating voltage that may be used bythe self-excitation circuitry 24. Thus, the self-excitation circuitry 24does not require a separate low-voltage power source, a great advantagein the power line context since it is difficult to provide and maintaina separate low-voltage power source on the power line 82.

An alternate approach is to power the sensor by using a coil 95 tocouple to the magnetic field associated with the current on the powerline. The problem with this approach is that the current drops as loadsare served along the length of the power line. On the other hand, theelectric field remains constant along the length of the power line.

FIG. 5 also illustrates that the MEM power line system housing 90supports a connection mechanism 98 for electrical and physicalconnection to the power line 82. In addition, the MEM power line systemhousing 90 supports a printed circuit board 96.

FIG. 6 is an axial view of the MEM power line attachment assembly 86.The connection mechanism 98 may be implemented with any widely knownpower line clamping device. Preferably, the connection mechanism 98includes an extending region 98' with an aperture (not shown). As knownin the art, the extending region 98' may be manipulated by a worker, orby a tool held by a worker, to position and lock the connectionmechanism 98 onto the power line 82.

By way of example, the connection mechanism 98 may be formed ofaluminum. A soft electrical conductor, such as a carbon motor brush mayalso be used for electrical connection to the power line. The advantageof this approach is that it will not damage the power line conductor andit will not lead to electrochemical corrosion effects. Protective greasemay be used around the connection mechanism 98 to prevent corrosion inair.

FIG. 6 illustrates that a protective housing 102 may be used over theprinted circuit board 96. In the embodiment of FIG. 6, the output deviceis implemented as an antenna 104.

Since the MEM attachment assembly 86 does not require a groundconnection, it is relatively easy to install. The ease of installationis facilitated by the compact size of the components associated with theassembly 86. The assembly 86 is relatively inexpensive and maintenancefree.

FIG. 7 illustrates a schematic of a printed circuit board 96 which maybe used in the MEM power line attachment assembly 86. The board 96includes the previously described MEM device 22, which may be housed ina standard integrated circuit housing. FIG. 7 illustrates the MEM plateinput node 32 and the MEM plate common node 34 connections. The MEMplate common node 34 is electrically connected to the connectionmechanism 98, while the MEM plate input node 32 is electricallyconnected to the first field plate 92. In a preferable embodiment, thefirst field plate 92 includes three electrically isolated metal regions.In such a configuration, one metal region is used to generate anelectrostatic voltage input signal for input node 32, and the othermetal regions are used to generate an electrostatic voltage input signalfor the power circuit 110, as will be described below.

The connections between the self-excitation circuitry 24 and the MEMdevice 22 are the same as shown in FIG. 1. FIG. 7 illustrates a MEMdevice output line 51 connecting MEM device 22 and output device 26. Aswill be shown in relation to FIG. 8, the MEM device output line 51 maybe connected to the drive line 50 from the self-excitation circuitry 24.

FIG. 7 illustrates a power circuit 110 with four input nodes. Asindicated above, preferably field plate 92 includes three electricallyisolated metal regions, two of which are used for the power circuit 110.In such a configuration, input nodes 112A and 112B would be respectivelyconnected to the two metal regions. Input nodes 114A and 114B arerespectively connected to two metal regions of field plate 94. The powercircuit 110 processes the voltage potential differences from the twosets of field plates to charge capacitors, whose voltage is used topower the self-excitation circuitry 24.

FIG. 7 illustrates a MEM device 22A connected to a coil 95. The coil 95is positioned such that the circumferential magnetic field lines of thepower line thread through the coil. A voltage proportional to thederivative of the current may then be obtained. Thus, the MEM device 22Amay be used to process current information associated with the powersignal on the power line. Instead of using a separate MEM device 22A, aswitch can be used to apply the output of the coil 95 to the MEM device22.

FIG. 8 provides a detailed schematic of several of the componentsmounted on the printed circuit board 96. The top of the figure shows avoltage signal applied to the MEM plate input node 32. The top of FIG. 8also shows the feedback line 48 connected between the MEM device 22 andthe self-excitation circuitry 24. The self-excitation circuitry 24 ofthe embodiment of FIG. 8 includes a high-pass filter 132 and a low-passfilter 136, which in combination define a resonant frequency passband ofinterest. Note that the self-excitation of the cantilever 28 is achievedby simple and inexpensive amplifiers 134 and 138 and that the cantileverresonant frequency signal can be easily obtained from line 50.

The output of the self-excitation circuitry 24 is drive line 50, whichis connected to MEM device 22. The drive line 50 also has an antenna 26connected to it. In this embodiment, the antenna 26 serves as the outputdevice. The antenna receives the variable frequency voltage signalsproduced by the MEM device. Typically, these signals will be in therange of ten to a few hundred kHz. The variable frequency voltagesignals from antenna 26 are then passed to a remotely located antennathrough electromagnetic induction. This direct coupling approach ishighly advantageous because it eliminates undesirable power drainsassociated with modulators and other radio components.

FIG. 8 also illustrates a current signal applied to a MEM device 22A.The current signal is from coil 95. The remaining processing of thesignal is as described in the previous paragraph.

Finally, FIG. 8 illustrates a power circuit 110. As shown, the powercircuit 110 may be implemented as two full-wave rectifier circuits. Thepower circuit input nodes 112A and 112B are electrically connected toelectrically isolated metal regions of the first field plate 92. Thepower circuit input nodes 114A and 114B are electrically connected toelectrically isolated metal regions of the second field plate 94. Thesetwo pairs of input signals are rectified by the bridge circuits BR1 andBR2. The output current of approximately 60 microamperes from each setof field plates is used to charge capacitors C1 and C7. This charge maythen be used by the amplifiers of the self-excitation circuitry 24 and24A to induce self-oscillation in the cantilevers 22 and 22A.

The disclosed configuration has been used to derive approximately 500microwatts from the electric field near a power line operating at 6kilovolts. The self-excited MEM device of the invention only requires afew hundred microwatts to operate.

Turning now to FIG. 9, illustrated therein is one embodiment of a MEMbase station 88 which may be used in accordance with the invention. TheMEM base station 88 includes an input device 150. The input device maybe an antenna to receive the electromagnetic induction drive signal froman antenna on the MEM power line attachment assembly 86.

In either embodiment, the input device receives a time-varying frequencysignal that is largely noise immune. The frequency of the signal is usedto reconstruct the characteristics of the voltage and current waveformsof the power line. FIG. 10 illustrates the experimentally derivedrelationship between a resonant frequency value of a MEM device and theelectrostatic field applied to the MEM device. In turn, theelectrostatic field value has a known relationship with the line-to-linevoltage or line current of the power line. Thus, the time-varyingfrequency signal from the MEM device 22 of the MEM power line attachmentassembly 86 may be used by the MEM base station 88 to reconstruct thevoltage and current waveforms of the power line. This data processing isconveniently performed at the base station 88 where an independent powersupply is typically available.

The processing of the frequency signal into corresponding voltage andcurrent waveforms of the power line may be performed in a variety ofways. One implementation is shown in FIG. 9. In the embodiment of FIG.9, the signal is applied to a frequency processor which includescounters 152, 154, clock 156, divide-by-two circuit 158, and multiplexer160. The signal at node 151 is applied in parallel to a first counter152 and a second counter 154. The second counter 154 is toggled by aninverted clock signal from clock 156, while the first counter 152 istoggled by the clock signal. This allows one counter to operate whilethe other counter holds the accumulated count from the previous sampleinterval. The outputs of the counters are connected to a two channelmultiplexer 160. The select line of the multiplexer is driven from theclock signal applied to the counter gates, making the accumulated countof the previous sample interval available at the output of themultiplexer 160.

The output of the multiplexer 160 may be connected to the address linesof a programmable read-only memory (PROM) 162 which serves as aconversion circuit. The output enable control line of the PROM 162 isconnected to the clock 156. Through the operation of the divide-by-twocircuit 158A, appropriate settling time of the data applied to theaddress input lines of the programmable read-only memory 162 is providedbefore the data output lines are enabled.

The output of the PROM is digital data which reconstructs the sensedpower signal. In other words, the variable frequency voltage signalresults in a digital address signal. The digital address signal accessesinformation in the PROM 162 defining the power line signal. For example,the frequency signal accesses a voltage signal derived from the electricfield that is proportional to the line-to-line voltage of the powerline. This information can then be transmitted to a central computingarea for comprehensive monitoring of a power line. In the alternative,the information can be stored for subsequent processing. In anotherembodiment, the information can be compared to a set of predeterminedthreshold values. If the comparison identifies a problem, a visual oraudio alarm may be activated at the MEM base station 88, or an alarmsignal may be sent to the power system control center.

The data mapping provided by the PROM 162 can be used to providecalibration data for the MEM device 22. Since the PROM 162 provides aone-to-one mapping from the applied address to the data output, complexcalibration calculations can be done once during calibration andembedded in the PROM 162, rather than being calculated in real-time.

The PROM 162 can also be programmed with a more involved data mapping.For example, if the signal voltage is known to be sinusoidal and RMSvalues of the voltage are required, the PROM 162 can be programmed withthe RMS value of the sinusoidal signal. A simple digital system can thenmonitor the digital data stream and store the peak values. This willprovide real-time RMS voltage data without requiring multiplications.

When voltage and current are monitored by the MEM power line attachmentassembly 86, the MEM base station 88 can be used as a watt or watt-hourmeter. In this embodiment, the output of the voltage and current signalsis combined to generate the address applied to the PROM 162. The datacontained in the PROM 162 would then be the power (watts) of the appliedvoltage and current or the energy consumed over the sample interval(watt-hours).

FIG. 11 illustrates one feasible embodiment of a watt or watt-hour meterin accordance with the invention. The meter 164 of FIG. 11 is connectedto a feeder line 166 and a feeder line 165, which receives stepped-downvoltage from a power line (not shown). A resistor network 167 is used asa voltage divider. Nodes 168 and 169 respectively apply a scaled voltagefrom the resistor network 167 to MEM device and processing circuitry170.

The meter 164 also includes a coil 172 positioned around feeder line 165and feeder line 166, the output of which is applied to the MEM deviceand processing circuitry 170 via nodes 174 and 176.

The meter 164 also includes a power supply 190 to energize the MEMdevice and processing circuitry 170. The power supply 190 may include astep-down transformer and a rectifier.

FIG. 12 illustrates one embodiment of the MEM device and processingcircuitry 170. In this embodiment, a switch 180 is used to apply thesignals from nodes 168 and 169 to MEM device 22 and then to apply thesignals from nodes 176 and 174 to MEM device 22. This configurationallows just one MEM device 22 to be used. However, a more accurateconfiguration incorporates two MEM devices 22 and eliminates switch 180.

The operation of the MEM device 22, self-excitation circuitry 24,counters 152 and 154, clock 156, divide-by-two circuit 158, andmultiplexer 160 operate as described above. The output of themultiplexer 160 is applied to a microprocessor 180, which combines theincoming frequency information relating to the feederline current andvoltage to obtain an address for PROM 182. The PROM 182 storesfrequency-to-watt conversion information that is accessed by themicroprocessor 180. The resultant watt information may be combined withtime interval information to obtain watt-hour information. The wattinformation and watt hour information may be stored in memory 184. Inaddition, the power information may be passed to interface devices 186,which may include a visual display of the information. The interfacedevices 186 may also include wire feeds, radio feeds, etc. to a centralcomputer responsible for processing metering information.

FIG. 13 illustrates a signal resolution circuit that may be used at thefront end of the MEM base station 88, shown in FIG. 9. The resolution ofthe self-excited MEM device 20 of the invention is a function of themagnitude of the frequency shift for a given applied voltage. Thegreater the frequency shift, the more accurately the applied voltagesignal can be estimated. In the apparatus of FIG. 13, the variablefrequency voltage signal from the input device 150 is converted from asinusoidal signal into a rectangular wave with a limiter 200, a knowndevice. This results in the creation of odd harmonics of the fundamentalfrequency which can be recovered with band pass (BP) filters 202 and204. The odd harmonic signals may be processed by limiters 206 and 208.One or more of the odd harmonic signals may then be applied to inputnode 151 of the processing circuitry of FIG. 9.

An important feature of the harmonic signals is that the higher theharmonic, the greater the signal resolution because of the increase infrequency change over the fundamental signal. A cantilevervoltage-to-frequency converter in accordance with the invention may havea resonant frequency that varies from 15 kHz to 10 kHz with an appliedsignal voltage of 0V to 10V. This results in a change in frequency ofapproximately 500 Hz per applied volt. The 3rd harmonic of this signalwill vary from 45 kHz to 30 kHz for the same applied voltage of 0V to10V. This will result in a change in frequency of approximately 1500 Hzper applied volt, a threefold increase in resolution, provided the noiseis constant.

FIG. 14 illustrates an alternate embodiment of the self-excitedmicroelectromechanical device 20A of the invention. The cantilever 28 ispositioned on a shim 213 within a housing 210. This embodiment of thedevice does not have a MEM plate 30, as shown in FIG. 1. Instead, ametallic plate lid 212 is provided on the housing 210. The MEM plateinput node 32 of FIG. 1 is substituted with a plate input node 32A,which receives the signal from the power line or other signal to bemeasured. A plate common node 34A is connected to the cantilever common40, as previously described. The operation of the resultant electricfield on the cantilever 28 is as previously described.

FIG. 15 illustrates another embodiment of the invention which does notrequire a MEM plate 30. In this embodiment, the cantilever 28 ispositioned within package 214. The package 214 includes a metallic platebase 218 with an electrically conducting spacer 220 protruding towardthe cantilever 28. In this configuration, the plate input node 32Bconnected to the metallic plate base 218 receives the signal to bemeasured. The electrically conducting spacer 220, by virtue of itsconnection to metallic plate base 218, creates an electric field whichdeflects the cantilever 28, as previously described.

The embodiment of FIG. 16 is similar to that of FIG. 15, but it does notinclude the spacer 220. Instead, the cantilever support frame 29 isthinned so that the cantilever is closer to the metallic plate base 226.

The self-excited MEM devices 20 of the invention may be used to monitorsuch physical phenomena as temperature or moisture. That is, thefrequency oscillation of the cantilever will vary in an empiricallydeterminable way in the presence of changing temperature or moisture.Thus, it may be desirable to provide an extra self-excited MEM device 20on a MEM power line attachment assembly to monitor the temperature ofthe conductor. This temperature information can be used to monitorphysical line sag or to correct the other measured power lineparameters, such as shaking of the conductor.

Those skilled in the art will recognize a number of benefits associatedwith the invention. The self-excited MEM device 20 provides enhancedsensitivity in the measurement of a variety of physical phenomena. Whenimplemented to measure an electric field, for example as a power linemonitor, the device of the invention allows for a compact configurationthat is powered solely from the electric fields surrounding the powerline. Thus, the device is relatively inexpensive, easy to install, andhas low maintenance requirements. When implemented in a power meter, theself-excited MEM device 20 allows for a compact, reliable, inexpensivesolid-state power meter.

The foregoing descriptions of specific embodiments of the presentinvention are presented for purposes of illustration and description.They are not intended to be exhaustive or to limit the invention to theprecise forms disclosed, obviously many modifications and variations arepossible in view of the above teachings. The embodiments were chosen anddescribed in order to best explain the principles of the invention andits practical applications, to thereby enable others skilled in the artto best utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the followingclaims and their equivalents.

We claim:
 1. A microelectromechanical device, comprising:a resonatingstructure which responds to a physical phenomenon by resonating at aresonant frequency, said resonating structure producing a variablefrequency voltage signal corresponding to said resonant frequency; andself-excitation circuitry connected to said resonating structure toprocess said variable frequency voltage signal and induce saidresonating structure to resonate in a resonant pass band of interest;wherein said resonating structure is a cantilever and saidmicroelectromechanical device further comprises a plate positionedadjacent to said cantilever so as to define a device gap therebetween;and wherein a voltage is applied to said plate, thereby generating anelectric field in said device gap that alters the mechanical forces onsaid cantilever to produce said variable frequency voltage signal. 2.The microelectromechanical device of claim 1 wherein said cantileverincludes a piezoelectric thin film positioned between a top metal layerand a bottom metal layer.
 3. The microelectromechanical device of claim2 wherein said top metal layer includes a first electrode and a secondelectrode, and said bottom metal layer operates as an electrical common.4. The microelectromechanical device of claim 2 wherein said bottommetal layer includes a first electrode and a second electrode, and saidtop metal layer operates as an electrical common.
 5. Themicroelectromechanical device of claim 1 wherein a feedback electrode ofsaid resonating structure electrically connects an input signal to saidself-excitation circuitry and said self-excitation circuitry produces anoutput signal that is electrically connected to a drive electrode ofsaid resonating structure.
 6. The microelectromechanical device of claim1 wherein said resonating structure includes a calibration electrode. 7.The microelectromechanical device of claim 1 wherein saidself-excitation circuitry includes a high-pass filter and a low-passfilter to limit said variable frequency voltage signal to said resonantfrequency pass band of interest.
 8. A microelectromechanical device,comprising:a resonating structure which responds to an electric field byresonating at a resonant frequency, said resonating structure producinga variable frequency voltage signal corresponding to said resonantfrequency; self-excitation circuitry connected to said resonatingstructure to process said variable frequency voltage signal and inducesaid resonating structure to resonate in a resonant pass band ofinterest; and a package enclosing said resonating structure, saidpackage including a metal portion to receive an input signal and inducesaid electric field adjacent to said resonating structure and therebygenerate said variable frequency voltage signal.
 9. Themicroelectromechanical device of claim 8 wherein said metal portion isincorporated into a lid for said package.
 10. The microelectromechanicaldevice of claim 8 wherein said resonating structure is positioned onsaid metal portion.
 11. The microelectromechanical device of claim 8wherein said metal portion includes an electrically conducting spacerextending toward said resonant structure.